1. Field of the Invention
The invention relates to a sensor device that detects a rotation angle, torque, or the like, of a detection object.
2. Discussion of Background
A device described in EP1503184 A2 is conventionally known as a sensor device of this type. The sensor device described in EP1503184 A2 includes a detecting unit that detects a rotation angle of a steering shaft of a vehicle. The detecting unit outputs a signal +sin θ that sinusoidally varies according to a rotation angle θ of the steering shaft and a signal +cos θ that cosinusoidally varies according to the rotation angle θ of the steering shaft. The detecting unit also outputs a signal −sin θ that is shifted in phase by 180° with respect to the signal +sin θ and a signal −cos θ that is shifted in phase by 180° with respect to the signal +cos θ. The four signals that are output from the detecting unit are input to a differential amplifying unit. The differential amplifying unit generates a first differential amplification signal (2 sin θ) by amplifying the difference between the signal +sin θ and the signal −sin θ, and outputs the first differential amplification signal to a computing unit. In addition, the differential amplifying unit generates a second differential amplification signal (2 cos θ) by amplifying the difference between the signal +cos θ and the signal −cos θ, and outputs the second differential amplification signal to the computing unit. The computing unit obtains the rotation angle θ of the steering shaft by computing an arctangent of these first differential amplification signal and second differential amplification signal.
On the other hand, in the sensor device described in EP1503184 A2, a first addition signal is generated by adding the signal +sin θ to the signal −sin θ, and a second addition signal is generated by adding the signal +cos θ to the signal −cos θ. When any one of the first addition signal and the second addition signal exceeds an upper limit threshold or becomes lower than a lower limit threshold, it is determined that there is an abnormality in the detecting unit. According to this abnormality detection method, it is possible to detect an abnormality in the detecting unit.
In the sensor device described in EP1503184 A2, for example, when the first addition signal exceeds the upper limit threshold, the first differential amplification signal has an abnormal value, and therefore, it is not possible to compute the rotation angle θ of the steering shaft. In addition, when the first addition signal exceeds the upper limit threshold, it is understood that any one of the signal +sin θ and the signal −sin θ is abnormal, however it is not possible to determine which one of the signal +sin θ and the signal −sin θ is abnormal. If a factor that makes the first addition signal exceed the upper limit threshold is an abnormality of the signal +sin θ and then it is possible to identify this factor, it is possible to continue computation of the rotation angle θ of the steering shaft using the normal signal −sin θ and the signal +cos θ or the signal −cos θ. Thus, redundancy significantly improves. Therefore, a sensor device that is able to identify a detection signal having an abnormality is desired.
The above-described problem is not limited to the sensor device that detects the rotation angle of the steering shaft, and it is a problem common to various sensor devices that detect a state quantity of any detection object, such as a sensor device that detects a torque that acts on the steering shaft.